In general, the washing machine is an appliance which removes dirt stuck to laundry by chemical decomposition and mechanical impact. As not only a total height of the appliance can be reduced compared to the pulsator washing machine of which inner tub rotates in a vertical position, but also a washing capacity can be increased more, and causes almost no problems, such as entangling of laundry, demand for the drum type washing machine increases day by day.
A related art structure of a drum washing machine will be explained, with reference to the attached drawing in detail. FIG. 1 illustrates a section of a related art drum washing machine, provided with a tub 2 supported by dampers 8 and springs 7 in a body 1, a cylindrical drum 3 rotatably fitted in the tub, and a driving part shaft coupled to the drum.
The driving part is provided with a rotor 4 and a stator 5 in rear of the tub 2, wherein the rotor 4 is directly coupled with a washing shaft 6 fitted to be rotatable with the drum for transmission of a driving force of the rotor to the drum 3 directly without a pulley or belt. There are a door 9 in a front face of the body opposite to an opening of the drum, and a gasket 10 between the door and the drum for sealing. In the foregoing drum washing machine, the rotation force of the rotor 4 is transmitted to the drum 3 through the washing shaft 6, and the washing is progressed as the laundry is lifted by a baffle 3a and dropped by gravity during rotation of the drum.
In the meantime, when the drum washing machine is transported, the tub 2 is shaken in up and down, and left and right directions because the tub 2 is supported by the springs 7 and the dampers 8, giving impacts to the driving part under the tub 2 or damage to the body 1. In order to prevent this, a device for fastening the tub 2 to the body in transportation is provided, which has a plurality of fastening bosses 11 on a rear surface of the tub and a plurality of fastening bolts 12 for passing through inserting holes in the body and coupled to the fastening bosses.
FIG. 2 illustrates a section of a key part of a related art fastening device for transportation of a drum washing machine, showing an enlarged view of the fastening bolt fastened to the fastening boss.
Referring to FIG. 2, since the fastening bolt 12 has an appropriate length of male thread part 12a on an outer circumference of an end thereof, and the fastening boss 11 has a female thread part 11a in an inner circumference thereof for engagement with the male thread part 12a, the fastening bolt and the fastening boss are fastened as the male thread part 12a and the female thread part 11a are engaged.
When the fastening bolt 12 is fastened to the fastening boss 11, a bolt holder 13, surrounding an outer circumference of the fastening bolt, protects the fastening bolt and maintains a gap between the tub and the body.
The impact occurred during transportation of the drum washing machine applies vibration to the tub 2, or to the fastening bolt 12 and the fastening boss 11, directly. In this instance, the vibration of the tub 2 acts as a bending moment or a shearing force to the fastening bolts 12 at a part the fastening boss 11 is in contact with the bolt holder 13. However, since the male thread part 12a on the fastening bolt 12 is exposed to outside of the fastening boss 11, there has been a problem in that the fastening bolt 12 becomes weak to strength.
That is, because the male thread part 12a on the fastening bolt is exposed outside of the fastening boss 11 in a state the fastening bolt 12 is engaged with the fastening boss 11 in the related art, there has been a stress concentration at a root part of the male thread part 12a in a part the fastening boss 11 is in contact with the bolt holder 13 due to the vibration or impact occurred during transportation of the drum washing machine, to cause bending in the root part of the male thread part 12a at which the stress is concentrated, with a high risk of breakage when the impact is excessive.
In other words, the root part of the male thread part of the fastening bolt at a point the fastening boss 11 and the bolt holder 13 are in contact acts as a notch that causes stress concentration, to weaken a strength of the fastening bolt against an impact to the tub.
Consequently, for solving the foregoing problem, it has been required to employ a thick fastening bolt with an adequate root diameter, or to provide a separate protection device.